Burner with adjustable flue gas recirculation

ABSTRACT

The invention relates to a burner (B) for firing a combustion chamber, having a supply duct (Z 1 ) for supplying combustion gas or combustion air into the combustion chamber, wherein the burner (B) has an inlet opening (E) via which flue gas ( 4 ) can flow into the supply duct (Z 1 ). To be able to adjust the flow rate of the flue gas ( 4 ) flowing into the supply duct (Z 1 ), the burner (B) comprises an adjustable throttle device (D).

The invention relates to a burner for firing a combustion chamber, with a feed duct for feeding fuel gas or combustion air into the combustion chamber, wherein the burner comprises an inlet opening via which flue gas may flow into the feed duct.

A burner of the above type is known, for example, from patent DE10337526. Said burner makes it possible to lower the maximum combustion temperatures through the recirculation of flue gases and thereby to reduce thermal nitrogen oxide formation. To this end, the burner, which is arranged at the bottom of the combustion chamber of a combustion furnace, comprises a first feed duct for feeding fuel gas into the combustion chamber, with an inlet opening for flue gas which opens towards the combustion chamber. The flue gas is sucked in by the pressurized fuel gas flowing into the first feed duct and mixes with the fuel gas. Combustion air can be introduced into the combustion chamber via a second feed duct. The outlet openings of the two feed ducts are arranged adjacent one another, such that the combustion air mixes with the fuel gas and the flue gas outside the burner.

Flue gas intake by the burner has an effect on the shape of the developing flame, which may for example be compressed or otherwise deformed, such that the reaction zone is concentrated onto a smaller spatial region and the maximum combustion temperature increases. This effect neutralizes, or at least lessens, the nitrogen oxide formation-reducing effect of the flue gas recirculation. Furthermore, a deformed flame may come too close to internals, such as reformer or cracking tubes, arranged in the combustion chamber and damage them.

The object of the present invention is accordingly to modify a burner of the above type in such a way as to overcome the outlined disadvantages of the prior art.

This object is achieved in that the burner comprises an adjustable throttle apparatus, by means of which the mass flow rate of the flue gas flowing into the feed duct is adjustable.

Depending on the position of the throttle apparatus, if the burner setting is otherwise unchanged a larger or smaller mass flow rate of flue gas may be sucked out of the combustion chamber. In this way, the flow ratios in the combustion chamber and thus the conditions for nitrogen oxide formation are also influenced.

The throttle apparatus is preferably constructed such that it is operable from outside the combustion chamber when the burner is in the installed state. With its assistance, the magnitude of the mass flow rate of the flue gas flowing into the feed duct may be varied at any time, but in particular during burner operation.

The burner according to the invention may be used to fire steam reformers and crackers, and also other process furnaces. Such apparatuses are operated in a steady state over relatively long periods. This means that the burner settings remain unchanged after ignition thereof and any heating phase which may follow thereafter. Since under such operating conditions the position of the throttle apparatus only occasionally needs to be changed, it makes sense for an apparatus to be provided which enables manual adjustment of the position thereof.

In a variant of the burner according to the invention, the throttle apparatus is connected to an adjusting drive, by means of which it is adjustable with the assistance of a motor. This variant for example allows the position of the throttle apparatus to be changed by remote control from a control desk. The throttle apparatus may furthermore be incorporated into a control loop, with which the rate of nitrogen oxide formation in the combustion chamber may be kept constantly at a predetermined value even when operating conditions vary over time.

It may be sensible, particularly when the throttle apparatus is part of a control loop, for the mass flow rate of the flue gas flowing into the feed duct to be varied in a linear or at least approximately linear manner with the position of the throttle apparatus. In one configuration of the invention, therefore, the inlet opening and/or the throttle apparatus are constructed with a shape by means of which, when the burner is in operation, an at least linear relationship arises between the position of the throttle apparatus and the variation of the mass flow rate of the flue gas flowing into the feed duct.

During burner operation, the throttle apparatus usually comes into contact with aggressive gases, the temperatures of which may be far greater than 1000° C. To ensure its functionality even over relatively long periods, it is necessary to construct the throttle apparatus with materials which are largely insensitive to the hot combustion chamber atmosphere. The throttle apparatus therefore conveniently consists wholly or partially of refractory material and/or ceramic fibers and/or a high temperature metal.

In a preferred configuration of the burner according to the invention, the throttle apparatus comprises a disk arranged so as to be movable lengthwise in the feed duct, the cross sectional shape of which disk largely corresponds to the shape of the clear cross section of the feed duct and which narrows the inlet opening to a greater or lesser extent depending on its position.

In another preferred configuration of the burner according to the invention, the throttle apparatus comprises a rotary slide valve mounted rotatably about the longitudinal axis of the feed duct, rotation of which may narrow the inlet opening to a greater or lesser extent.

The feed duct preferably comprises a duct for feeding fuel gas into the combustion chamber. This variant allows the feed duct to be compactly constructed, since the relatively significant momentum of the inflowing fuel gas may be used to suck flue gas in effectively.

The invention will be explained in greater detail below with reference to an exemplary embodiment illustrated schematically in FIG. 1.

FIG. 1 shows a longitudinal section through a preferred variant of the burner according to the invention.

The burner B, which is arranged at the bottom A of a combustion chamber, comprises a first feed duct Z1 for supplying combustion fuel 1 and a second feed duct Z2, via which combustion air 2 may be fed into a combustion chamber. In the first feed duct Z1 a gas feed tube R is arranged, which is connected with a fuel gas source under overpressure (not shown) and by means of which fuel gas 3 is expandable into the first feed duct Z1. A hole C is provided in the side wall of the first feed duct Z1, which hole opens towards the combustion chamber and through which flue gas 4 may flow in from the combustion chamber, to form a gas mixture 5 with the combustion gas 3. Through the outlet opening F1 the gas mixture 5 can flow out into the combustion chamber, wherein it mixes with combustion air 6, which latter leaves the second feed duct Z2 via the outlet opening F2, and forms a reactive gas mixture. In the first feed duct Z1 a throttle apparatus D is arranged, which comprises a disk P and a rod S connected firmly therewith. The disk P, the cross section of which corresponds largely to the cross section of the first feed duct Z1, is vertically mobile. The rod S is mounted in a mobile manner in the bottom of the burner B and passed outwards therethrough. This makes it possible to manipulate the rod S from outside the burner B, in order thereby to change the vertical position of the disk P in the first feed duct Z1. The cross section of the inlet opening E, and thus also the flow resistance for the flue gas 4 flowing into the first feed duct Z1, varies depending on the position of the disk P, such that it is possible to adjust the mass flow rate of the flue gas 4 with the assistance of the throttle member D. 

1. A burner (B) for firing a combustion chamber, with a feed duct (Z1) for feeding fuel gas or combustion air into the combustion chamber, wherein the burner (B) has an inlet opening (E), via which flue gas (4) may flow into the feed duct (Z1), characterized in that it comprises an adjustable throttle apparatus (D), by means of which the mass flow rate of the flue gas (4) flowing into the feed duct (Z1) is adjustable.
 2. The burner as claimed in claim 1, characterized in that the throttle apparatus (D) is operable from outside the combustion chamber when the burner is in the installed state.
 3. The burner as claimed in claim 1, characterized in that the throttle apparatus (D) is operable manually or by means of an adjusting drive.
 4. The burner as claimed in claim 1, characterized in that the throttle apparatus (D) consists wholly or in part of refractory material and/or ceramic fibers and/or a high temperature metal.
 5. The burner as claimed in claim 1, characterized in that the throttle apparatus (D) comprises a disk (P) arranged so as to be movable lengthwise in the feed duct (Z1), the cross sectional shape of which disk largely corresponds to the shape of the clear cross section of the feed duct (Z1) and which narrows the inlet opening (E) to a greater or lesser extent depending on its position.
 6. The burner as claimed in claim 1, characterized in that the throttle apparatus (D) comprises a rotary slide valve mounted rotatably about the longitudinal axis of the feed duct (Z), rotation of which may narrow the inlet opening (E) to a greater or lesser extent. 